In the case of an industrial installation, it is frequently necessary to provide a means for locking a rotatable operating member, either to secure that operating member against unauthorised operation or to prevent its operation other than a the correct point in a sequence of other associated operations. For example, fluid-flow control valves often have to be secured in this way.
A valve operating member may be secured against unauthorised operation by a simple chain or padlock but more recently there have been proposals for much more sophisticated mechanisms adapted for mounting on a valve, preferably without the need to modify the valve body in any way. Such a mechanism typically is key-operated and arranged so that the key must be inserted to permit operation of the valve. Moreover, the key usually is held trapped until the mechanism is locked to prevent valve operation, and such locking ordinarily is possible only when the valve has been operated to some pre-determined setting--notionally either one or both of fully opened or fully closed. It is usual to refer to such an arrangement as a "lock mechanism".
When security is required to prevent a valve being operated out of turn within a pre-determined sequence of a plurality of valve operations, there have been proposals for so-called "interlock mechanisms", where two separate keys must both be engaged with a lock mechanism in order to allow the valve to be operated. The arrangement should be such that both keys must be present in the lock mechanism to permit valve operation and are held trapped until the valve has been set to one of its two possible extreme settings: one or the other key then may be removed depending upon which extreme setting has been reached. Sequencing of the valve operations may be obtained by arranging for the key which is removable at one extreme setting to be used in another similar interlock mechanism to permit operation of the valve fitted with that similar interlock, and so on.
It is relatively simple to design a lock mechanism or an interlock mechanism which is compact and efficient in its operation where the valve operating member has to be turned through a relatively small angle, from one extreme setting to the other--for example, 90' in the case of a ball valve. When however a lock mechanism or interlock mechanism is to be fitted to a valve the operating member of which requires a considerable number of turns to take the valve from one extreme setting to the other, then there are considerable problems in designing a mechanism which is able to operate in a satisfactory manner. In particular, steps must be taken to prevent the possibility of locking the mechanism at the same angular position on each turn of operating movement. The mechanism thus tends to become rather bulky, in order to accommodate the amount of movement required to take the valve from one setting to the other, and, particularly in the case of a rising-stem valve, the mechanism must provide adequate clearance for the rising stem itself. In addition, to allow one key or the other to be released at an extreme setting, the mechanism needs careful calibration and setting-up, including the machining of certain internal components to parameters determined by the valve characteristics to suit the precise number of turns required to take the valve from one extreme setting to the other. This however takes no account of the wear which inevitably occurs with a valve; thus, though the mechanism may function correctly when first installed, it may cease to function and may require the provision of replacement parts in order to allow its continued use on a valve after a period of service.